Vibration detector and method for rotating shaft

ABSTRACT

Apparatus for detecting abnormal vibration in a shaft, such as a rotating crucible holder drive shaft of a crystal puller, has a pair of sensors disposed 90 degrees with respect to each other. The signals are high pass filtered and added together, then low pass filtered and full wave rectified to operate an alarm and strip recorder. A method for detecting vibration in a shaft comprises sensing vibrations in the shaft, filtering the sensed signals, and rectifying the filtered signals.

This was made with Government support under DAAL 03-86-C-0022 awarded byU.S. Army Research Office. The government has certain rights in thisinvention.

This application is a continuation of application Ser. No. 07/370,667,filed Jun. 23, 1989, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a vibration detector for a rotatingshaft, and more particularly, to such a detector used with aCzochralski-type crystal puller.

In the Czochralski-type crystal puller, a melt of the crystal materialis disposed in a heated crucible, which is attached to a rotating shaft.Surrounding the heater is an insulating jacket called "heaterfurniture", and the whole apparatus is mounted on a baseplate. A seedcrystal is placed in the melt and pulled up, and some of the meltsolidifies on the seed in crystallographic alignment therewith. Thissolidified portion is called the "boule". When the crystal to be formedis GaAs, a very high pressure inert gas must be used to prevent the Asfrom vaporizing. The high pressure causes the gas to be a good thermalconductor. To prevent loss of heat through the gas, which would occur ifthe gas goes between the heater furniture and the crucible, tighttolerances are used between the rotating crucible and the heaterfurniture, in particular an insulating cap thereof. However, then thecrucible will sometimes make contact with the cap. If the contact ishard enough, it will cause the crucible to break out into a rotaryoscillation. This oscillation can cause failure to gain control ofcrystal growth, and therefore termination of the pull with shorter thandesired crystal length, twinning or dislocations in the crystal beforethe desired length is achieved, and breaking of the boule off the seedand its falling into melt. If boule breakage occurs, it can fracture thecrucible. This often causes catastrophic damage to the puller, and sincethe leaking melt is conductive and hot, this can result in a destroyedheater and even a partially melted baseplate. Further, all items thatcome in contact with the melt (except the crucible) become contaminatedwaste.

Presently it can only be determined if the crucible is in a rotationaloscillation by observing it on a video monitor. This is dependent onsufficient heat in the crystal chamber to adequately light the crucible,e.g., at least about 4 hours after start of heat-up. It is alsonecessary to have an operator present at the time of the oscillation andact promptly (typically within a few seconds) to correct it.

It is therefore an object of the present invention to have a warningsystem for vibration of rotating shaft that provides clear and earlywarning of the vibration.

SUMMARY OF THE INVENTION

Apparatus in accordance with the invention for detecting vibration of ashaft comprises at least a first non-contacting proximity sensor adaptedto be disposed proximate the shaft; filtering means coupled to saidsensor; and AC detection means coupled to said filtering means.

A method in accordance with the invention for detecting vibration of ashaft comprises sensing vibration in the shaft without contacting theshaft to provide a sensor signal; filtering said sensor signal; and ACdetecting the filtered signal.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a partly cross-sectional and partly block diagram of theapparatus in accordance with the invention;

FIG. 2 shows a detail of a block used in FIG. 1; and

FIG. 3 shows a strip recorder output graph.

DETAILED DESCRIPTION

As shown in FIG. 1 a crucible puller apparatus, generally designated 10,is largely conventional and can be Model 358 made by CambridgeInstruments, Cambridge, U.K., and thus will be only briefly described. Acylindrical sidewall 11 overlies a base plate 12 to form with a top (notshown) a pressure chamber. Plate 12 has a drive shaft 14 extendingtherethrough that is driven at its bottom end by drive apparatus 16.Apparatus 16 comprises both a motor (not shown) for rotation of shaft 14and another motor and worm gear (neither shown) for vertical linearmotion of shaft 14. Coupled to the upper end of shaft 14 is cruciblesupport rod hardware 18, which in turn supports a graphite crucibleholder 20. Disposed within holder 20 is a BNO₃ crucible 22 of betweenabout 0.015 to 0.020 inches (0.0381 to 0.0508 cm) thickness. Disposedaround crucible holder 20 is an electrical resistance heater 24, whiledisposed around heater 24 is heater furniture 26 comprising alternatinglayers of graphite and graphite blankets. An insulating furniture cap 28overlies heater furniture 26 and is considered a portion thereof. Theclearance gap 90 between cap 28 and holder 20 is very small to preventan inert gas (not shown), e.g., Ar, N₂, etc., at very high pressure fromflowing therebetween and causing loss of heat. This inert gas is used ata high pressure to prevent vaporization of product reagents, e.g., Ga,As, etc. Within crucible 20 is a melt 30 of a semiconductor material,e.g., GaAs, while above melt 30 is a protective layer 32 of, e.g., B₂ O,to prevent contamination of melt 30 by any O₂ that may be present in thechamber. A seed crystal 34 of the product to be grown, e.g., GaAs, hasgrown from it a boule crystal 36.

In operation, the seed crystal 34 is rotated and simultaneously pulledup by apparatus (not shown) as the boule crystal 36 grows, which tendsto lower the top level of melt 30. Simultaneously, apparatus 16 rotatesdrive shaft 14 in the opposite direction from that of crystals 34 and 36and moves it up. This upward movement is to keep the top level of melt30 at a constant level relative to heater 24, which has been found to becritical for good monocrystalline growth. However, due to the very smallgap 90 between cap 28 and holder 20, binding can occur therebetween andhence eventually oscillation of drive shaft 14, with the negativeresults described above, e.g., damage to the crucible, the crystal, andthe puller.

In accordance with the invention, two non-contacting proximitytransducer probes 40 and 42 are located around drive shaft 14 just belowbase plate 12. Sensors or probes 40 and 42 can be of the eddy currenttype, i.e., coils, such as the 7200 series made by Bentley-Nevada Corp.,Minden, Nev., or model KD 2400 made by Kaman Instrumentation Corp.,Colorado Springs, Colo. Other types of non-contacting sensors, e.g.,hysteresis, capacitance, photoranging, etc., can be used. In general,non-contacting sensors are used to limit motion pick up to that of shaft14, and their output signals require less filtering and signal analysisto determine shaft abnormalities. Non-contacting sensors provide a DCoutput signal with a nominal AC component when shaft 14 is justrotating, and an additional AC signal when shaft 14 is also vibrating.The output signals from sensors 40 and 42 are applied to a signalconditioner 43 (described below).

As shown in FIG. 2, transducer sensors 40 and 42 are preferably disposedat a 90 degree angle with respect to one another. If there is bindingbetween cap 28 and holder 20, there will be no change in the outputsignal from that transducer sensor which is at an angle of about 0 or180 degrees from the binding point. Thus the 90 degree arrangementensures an output signal from at least one transducer sensor. Ifdesired, three or more transducer sensors at mutually equal angles canbe used, or a single transducer sensor can be used to pick upvibrations, but it will not reliably pick up touching.

The analog signals from transducer sensors 40 and 42 are respectivelyapplied to transducers 40a and 42a and then passed through one Hzhigh-pass cut off frequency filters 44 and 46, respectively, and aresummed together in adder 48. In a particular embodiment, each of thefilters 44 and 46 comprises a series input capacitor and the shunt inputresistance of adder 48, and an additional two pole high pass filter witha 1 Hz cut off frequency in adder 48 for a total of three poles of highpass filtering. The filters 44 and 46 are used to eliminate the 0 to 30RPM (DC to 0.5 Hz) normal noise of rotating drive shaft 14. Theresulting signal from adder 48 is passed through a low-pass filter 50 ofapproximately 500 Hz cutoff to eliminate shaft pressure seal noise at afrequency of about 1.2 KHz caused by sequential stick and slip. In aparticular embodiment, filter 50 comprises a 6 pole active modifiedBessel filter for good pulse response. Details of designing such afilter can be found in "Transducer Interfacing Handbook" by AnalogDevices Co., Norwood, Mass. This high frequency stick and slip does notcause the damage that the above described binding does because itsfrequency is well above the resonant frequency of the entire pullingapparatus. Further, a narrower pass band has been found sometimesuseful, e.g., 70 to 200 Hz, and more particularly, 100 to 130 Hz. Thelower of these frequencies can be used in the filters 44 and 46, whilethe higher of these frequencies can be used in filter 50. It will beappreciated that filters 44 and 50, and also 46 and 50, comprise a bandpass filtering means. If desired, the output signals from transducers40a and 42a can be directly added and the added signal passed through aband pass filter with the lower and upper cut off frequencies givenabove.

The output signal from filter 50 is then full wave rectified by ACdetector or rectifier 52. A full wave rectifier is preferably used sothat motion of shaft 14 in either direction can be detected althoughother types of rectifiers can be used. In a particular embodiment,rectifier 52 was an active rectifier. The output signal from rectifier52 is a D.C. representation of crucible behavior and is applied to apeak detector 54 and from it to a chart recorder (not shown) fordisplay. In a particular embodiment, wherein the chart recorder had abandwith of 3 Hz, peak detector 54 had a time constant of 10 seconds.However, if the chart recorder has a wider bandwidth, then a lower timeconstant can be used. Further, peak detector 54 was of the active type.

The output signal from rectifier 52 is also applied to an alarm circuit56. Low level D.C. (approximately 50 mv) from rectifier 52 representsnormal crucible activity. Rapid increases in D.C. level are indicationsof abnormal crucible behavior. In the alarm circuit 56, if the full waverectified D.C. voltage exceeds a preset trip point value determined by apotentiometer 58, a retriggerable monostabile multivibrator on-shottherein (not shown), turns on a resettable audible alarm 60 and light62. If the fault was caused by a momentary contact, the alarm will soundfor approximately one second, but the light will stay on until theoperator resets it. In the event of a crucible oscillation, the alarm 60and light indicator 62 will remain on until the fault, i.e.,oscillation, is cleared and/or a reset switch (not shown) in alarm 56has been pressed.

FIG. 3 shows a chart recorder output calibrated in 24 hour time. Up toabout 0930 there is no binding. This is due to an initial alignment thatis performed before the puller starts operation. After 0930 some bindingtakes place as indicated by spikes 70. As time passes, the spikes becomelarger with a particularly large spike 72 at 1300, thus indicating thatthe binding is becoming harder. After 1330, several large spikes 74occur with a generally increasing amplitude. Finally, at about 1415, avery large continuous oscillation 76 occurs. An operator observing theoscillation 76 can clear the incipient fault by slowing down therotation of shaft 14 and then bring it back up to normal rotationalspeed to resume the normal growth rate of boule 36. Usually whenbringing shaft 14 back up to normal speed, the oscillation will notreoccur because the binding caused by grinding of cap 28 and/or holder20 increases the tolerance therebetween.

It will be appreciated the many other embodiments are possible withinthe spirit and scope of the invention. For example, an AC detector, suchas an AC voltmeter, can be coupled to the output of filter 50, or evendirectly to the output of adder 48, and the needle or digits of thevoltmeter watched by the operator for a rising average value. Thiseliminates the need for elements 50 to 62 of FIG. 2.

What is claimed is:
 1. Apparatus for detecting vibration of a rotatingshaft in a Czochralski type crystal puller said apparatus comprising:afirst non-contacting proximity sensor adapted to be disposed proximateto the rotating shaft in order to detect vibratory movement thereof; atleast a second non-contacting proximity sensor similarly disposed at apredetermined angle with respect to the first sensor in order to detectvibratory movement of said rotating shaft; high pass filtering meanscoupled to each of said sensors; an adder coupled to each of said highpass filtering means; low pass filtering means coupled to said adder;and AC detection means coupled to said low pass filtering means.
 2. Theapparatus of claim 1 wherein said sensors are disposed at about a 90degree angle with respect to each other.
 3. The apparatus of claim 1wherein said proximity sensor comprises an eddy current sensor.
 4. Theapparatus of claim 1 wherein said high pass and low pass filtering meanscooperate to provide a bandpass of between about 1 Hz to 500 Hz.
 5. Theapparatus of claim 1 wherein said high pass and low pass filtering meanscooperate to provide a bandpass of between about 70 Hz to 200 Hz.
 6. Theapparatus of claim 1 wherein said high pass and low pass filtering meanscooperate to provide a bandpass of between about 100 Hz to 130 Hz. 7.The apparatus of claim 1 wherein said detection means comprises a fullwave rectifier.
 8. The apparatus of claim 1 further comprising an alarmcircuit coupled to said detection means.
 9. The apparatus of claim 1further comprising a peak detector coupled to said detection means. 10.A method for detecting vibration of a rotating shaft in a Czocralskitype crystal puller, said method comprising:sensing vibration in theshaft with first sensing means adapted to provide a first sensor signalwithout contacting the shaft; sensing vibration in said shaft withsecond sensing means adapted to provide a second sensor signal withoutcontacting the shaft; high pass filtering the respective sensor signals;summing together said respective sensor signals in an adder means; lowpass filtering the summed signal; and AC detecting the summed andfiltered signal in order to identify vibrations endangering crystalgrowth.
 11. The method of claim 10 wherein said detecting step furthercomprises full wave rectifying.
 12. The method of claim 10 furthercomprising peak detecting the AC detected signal and displaying the peakdetected signal.
 13. The method of claim 10 further comprising providingan alarm signal when the AC detected signal exceeds a trip pointselected to correspond to said endangering vibrations.